FT-IR spectrometers are well known in the prior art. Such spectrometers may be used to identify compounds contained in a sample through recognition of the compounds, characteristic absorption of infrared radiation at various frequencies.
An FT-IR spectrometer is shown schematically in FIG. 1 and is generally indicated 10. The spectrometer includes an infrared source 12. Radiation from source 12 is collimated by a mirror 14. The resultant beam (shown in phantom) passes to and is divided by a beam splitter 16. Half the beam is passed to a fixed mirror 18, and half is directed to a moving mirror 20. Moving mirror 20 is movable in the directions of Arrow A as indicated by phantom outline.
The split beam is reflected by mirrors 18 and 20, recombines at beam splitter 16 and constructively or destructively interferes depending on the difference in the length of the optical paths between mirrors 18 and 20 and the beam splitter.
During operation of the spectrometer, moving mirror 20 moves at a velocity in one direction. As a result, a beam emerges from the beam splitter which is a mixture of modulated frequencies. This mixture is reflected off a mirror 22 onto a path 24. Path 24 passes through a window 26.
Window 26 supports a sample of material to be analyzed. Window 26 is mounted in a positioning apparatus 28, which enables the operator of the spectrometer to move the window so that the sample is in path 24. When the FT-IR spectrometer is operated in the transmittance mode shown in FIG. 1, the window is made from a material that allows radiation in the infrared range to pass therethrough without substantial absorption. Common materials for windows used in transmittance spectroscopy include potassium bromide and sodium chloride.
After the radiation passes through the sample on the window, it is reflected through mirrors 30 and 32, which focus the radiation on an infrared detector 34. Signals are generated by the infrared detector in response to the amplitude of the radiation sensed. These signals are sampled by the FT-IR spectrometer at predetermined intervals during movement of moving mirror 20. The spectrometer operates to plot the superimposed amplitudes of the radiation as a function of time. This produces an interferogram which is unique to the sample material.
The FT-IR spectrometer also has a He-Ne laser source 36. A laser beam from the laser source (shown in phantom) is reflected by a mirror 38 onto the beam splitter 16. The beam from the laser source is divided by the beam splitter and reflected to fixed mirror 18 and moving mirror 20. However, unlike the infrared radiation, the emerging beam from the laser is focused to a laser detector 40. The laser detector detects a modulated signal of constructive and destructive interference depending on the distances between mirror 20 and mirror 18 and the beam splitter. The spectrometer also plot the radiation sensed by the laser detector as a function of time to produce a reference interferogram.
The reference interferogram from the signals produced by the laser detector is used as a reference to convert the interferogram of infrared radiation via a Fourier Transformation into a plot of superimposed amplitudes as a function of frequency. The reference signals from the laser detector 40 are also used to provide feedback for controlling the speed of moving mirror 2 and timing for the sampling of signals from the infrared detector 34.
The FT-IR spectrometer also includes a display such as a CRT or plotter. The display produces a graphical representation of transmittance of the amplitude of the infrared radiation passing through the sample as a function of "wave numbers" which are inversely related to frequency. This graphical representation is characteristic of the compounds which comprise the sample. Through comparison to graphs of samples of known composition, the composition of the unknown sample may be determined.
In absorption spectroscopy the composition of the air in the path, and the material which makes up the window, may absorb radiation at certain frequencies. To compensate for this "background" absorption, it is often desirable after the sample has been scanned to move the positioning apparatus 28 and the window 26 so that the sample is no longer in the path. Another scan is then run with only the air and the window in the path. As a result a background scan is obtained.
The graphical representation of transmittance as a function of wave numbers, obtained in the background scan, may then be subtracted from the sample scan to provide a scan associated exclusively with the sample material. Many commercially available FT-IR spectrometers include means for doing this electronically. Such spectrometers are available from Nicolet, Inc. of Madison, Wis. Such commercially available spectrometers also typically include means for storing and comparing infrared scans of various known compounds with the scan produced by the sample for matching purpose and to determine the makeup of the sample.
A disadvantage of using an FT-IR spectrometer in the transmittance mode is that the window must be made of a material that passes light at most frequencies in the infrared range. However, many materials that have this characteristic react with items that are desired to be analyzed. This makes it difficult to do the analysis. In addition, no window material is perfect for transmitting radiation in the infrared range at all frequencies. As a result, this approach to sample analysis has limitations.
Many FT-IR spectrometers may also be operated to perform reflectance spectroscopy. FIG. 2 shows the spectrometer 10 of FIG. 1 modified to perform reflectance spectroscopy. All of the parts of the spectrometer are the same as in FIG. I except as noted below.
In reflectance spectroscopy the path 24 of the radiation in the infrared range impinges on a sample that is to be analyzed. The sample is supported on a window 42 which is mounted in a positioning apparatus 44. Unlike the window used when the spectrometer is operated in transmittance, window 42 is reflective to radiation. As a result the radiation bounces off window 42 and is passed through the sample a second time before finally passing to the detector. This double pass through the sample provides better, more accurate scanning and analysis in many cases.
However, reflectance spectroscopy using an FT-IR spectrometer has drawbacks. Because the windows are typically glass materials coated with a thin layer of silver or gold, the layer is very fragile and subject to being scratched or abraded during routine handling or cleaning. Damage to the coating may render a scan inaccurate and will require the window to be discarded.
The fragile nature of the window coating also gives each window a short useful life. Further, because the windows are coated with silver or gold, they are expensive. Some windows have been made with a reflective aluminum coating. While aluminum is less expensive, it has the drawback of being reactive with many sample materials.
A further problem with conventional windows used in reflectance spectroscopy is that it may be difficult to position the sample in the radiation path. Some conventional FT-IR spectrometers include a visual microscope with a sight that enables the operator to see the window and the sample on it so he can position the sample in the radiation path. Prior art windows are reflective of both visible light as well as infrared. It may be difficult to see the sample on the window due to an abundance of reflected light. As a result, it is difficult to position a sample to obtain a desired scan. The reflectivity of visual light also makes it difficult for the operator to see the surface of the window to insure that an area used for a background scan does not have imperfections in the coating. This adds uncertainty to the analysis. Other drawbacks associated with conventional windows used for reflectance spectroscopy include the difficulty of sample preparation. In some cases it may be desirable to pass an electric current under the sample being analyzed for purposes such as heating. This is not possible with conventional windows. In addition, silver or gold plated windows do not tend to hold sample materials well, but rather repel the samples causing them to move during handling. It is also difficult to apply items to be analyzed on conventional windows by methods such as condensation. This is because exposure to extremes of temperature or chemical compounds may damage the silver or gold coating. Also, it is sometimes desirable to review a sample in a conventional magnifying microscope. This is not possible for conventional windows which have a coating that is reflective of visible light.
Thus there exists a need for a window for use in reflectance spectroscopy with an FT-IR spectrometer that is more durable, lower in cost and easier to use than prior art windows.